A shadow mask is a parallax device for ensuring color purity in color cathode ray tubes, i.e. that beams from respective electron guns land only on respective different phosphors which emit different colors when activated by an electron beam. The pattern in which each phosphor is deposited on the screen face is identical to a pattern of apertures in the shadow mask but each pattern is slightly displaced from the others to allow each electron gun to see through the apertures only the phosphor it is intended to activate. The patterns usually found are sets of round holes, perpendicular rows of slits, such as in the precision in-line tube, or perpendicular slits extending from top to bottom of the screen, such as in the aperture grid tube.
A problem associated with shadow mask tubes is that of ensuring that beams from different guns coincide at that shadow mask. This ensures that corresponding elements of the phosphor patterns are activated simultaneously by the electron guns so that mixtures of primary colors can produce clearly defined secondary colors. If such coincidence does not occur, separate colors are visible The problem is called the convergence problem and is to some extent subjective, in that a certain degree of misconvergence is not visible to, or can be tolerated by, the viewer of a domestic television set, whereas the same degree of misconvergence would not be acceptable to a viewer, at perhaps one meter from the screen, using a color tube as a data display terminal.
Satisfactory correction of misconvergence in domestic television sets has been achieved by the provision of correction coils which enhance or diminish the effect of the main deflection coils in accordance with the position of the electron beams. The currents for the correction coils are derived directly or indirectly from the currents supplied to the main deflection coils. Such techniques have not yet proved satisfactory in meeting the more demanding standards of a data display terminal, nor have they been successfully applied to large (66 cm) television screens. Adjustments to the current supply consist in a complicated sequence of adjustments to a number of potentiometers and are impossible for the unskilled user.
Another approach is disclosed in British Patent Specification 1,517,119 (U.S. Pat. No. 4,203,051) and in British Patent Application 38584/77 (U.S. Pat. No. 4,203,054). Representations of the correction currents supplied to the correction coils are stored in a digital store, there being, in general, a different correction current for each of 256 areas of the screen. The digital representations of the correction currents are read from the store, and supplied to digital-to-analog converters, connected to the correction coils, synchronously with the scanning of the electron beams across the screen in a line raster. Should misconvergence occur, an operator can change the values in the digital store by operating a keyboard, observing the effect of the changes on test patterns displayed on the screen. It is unnecessary for the operator to correct misconvergence at all areas of the screen. The aforecited U.S. Pat. No. 4,203,054 application describes a technique whereby corrections made at only a few points can be extrapolated to the whole screen. It will be noted that the values held in the digital store directly represent the correction currents.
A disadvantage of the digital convergence correction technique outlined above is its cost. In a conventional three-gun shadow mask tube for each area of the screen, four correction factors are required, namely a correction factor for each beam, and a correction factor for the blue lateral coil. If the screen is divided into 256 areas, 1K byte of storage is required. This is a not insignificant overhead in the cost of a color display terminal. On the other hand, it is highly desirable to retain the advantage provided by the ease with which the user can adjust the convergence by means of the keyboard or some other easily manipulable input device.
Analog computing techniques for correction non-linearities in cathode ray tubes are known. British Patent Specification 1,066,643 discloses cathode ray tube apparatus in which, when it is intended to address the point (x,y) on the tube screen, beam deflection signals of magnitude x(1+Kr.sup.2) and y(1+Kr.sup.2) (K constant, r.sup.2 =x.sup.2 +y.sup.2) are computed by analog circuit means which inter alia determines log (xKr.sup.2) and log (yKr.sup.2). This is only one example of many such disclosures in the prior art.